1. Field of the Invention
The present invention relates generally to semiconductor wafer cleaning. More specifically, the present invention relates to a method and apparatus for using high-frequency acoustic energy with supercritical fluid to perform a semiconductor wafer cleaning operation.
2. Description of the Related Art
In the manufacture of semiconductor devices, a surface of a semiconductor wafer (“wafer” or “substrate”) must be cleaned to remove chemical and particulate contamination. If the contamination is not removed, semiconductor devices on the wafer may perform poorly or become defective. Particulate contamination generally consists of tiny bits of distinctly defined material having an affinity to adhere to the surface of the wafer. Examples of particulate contamination can include organic and inorganic residues, such as silicon dust, silica, slurry residue, polymeric residue, metal flakes, atmospheric dust, plastic particles, and silicate particles, among others.
Traditionally, wet-cleaning of a wafer has been performed using conventional solvents composed of aqueous, semi-aqueous, or organic solvent chemistries. In general, the conventional solvents can be applied to the wafer in the form of a bath or rinse. Some wafer cleaning processes also incorporate mechanical assistance from scrubbing brushes or high-pressure sprays. Also, most wet-cleaning processes are followed by a deionized water rinse and subsequent wafer drying process. Depending on the solvent used, the properties of both the solvents and the rinses used in the wet-cleaning process have a surface tension property with a wetting angle that is a function of the surface characteristics of the substrate. The surface may be hydrophilic, hydrophobic, or have properties somewhere in-between hydrophobic or hydrophilic. In cases where the surface is hydrophilic, a solution with a low wetting angle will easily wet the surface, and the fluid will be drawn into high-aspect ratio features by capillary forces. These capillary forces must be overcome to remove the liquid from the features after cleaning. Therefore, the high surface tension causes the liquid solutions to collect and adhere within structures present on the wafer surface, thus presenting difficulty during the drying process.
FIG. 1A is an illustration showing the collection and adherence of an aqueous or semi-aqueous solution 105 between high aspect ratio wafer structures 103 present on the surface of a wafer 101 following a wet-cleaning process, in accordance with the prior art. High surface tension causes the aqueous solution 105 to resist removal from between the structures 103 during the drying process. Since the aqueous or semi-aqueous solution 105 tends to be retained between the structures 103, removal of the solution 105 can cause the collapse of very small structures due to the capillary forces. Hence, there is a potential for the structures 103 to be damaged. This phenomenon is a well known issue in the cleaning and drying of MEMs structures, especially in MR heads.
FIG. 1B is an illustration showing distortion damage of the structures 103 caused by capillary force collapse during the drying process, in accordance with the prior art. As the aqueous or semi-aqueous solution 105 is removed by a high speed spin, evaporation, or other means, the high aspect ratio wafer structures 103 can be forced together as indicated by arrow 107 due to the surface tension caused by the capillary forces present. The distortion of the structures 103 can adversely affect subsequent wafer processing and ultimate device performance.
FIG. 1C is an illustration showing delamination damage of the structures 103 caused by capillary force collapse during the drying process, in accordance with the prior art. Again, as the solution 105 is removed from the high aspect ratio features, the capillary forces can cause collapse of the high aspect ratio wafer structures 103 to the point of delamination from an underlying substrate material as indicated by arrows 109. The propensity of the wafer structures 103 to delaminate is a function of the bond strength between the structures 103 and the underlying substrate material. Delamination damage of the structures 103 will certainly cause subsequent wafer processing and ultimate device performance to be adversely affected.
Aqueous or semi-aqueous solutions used for conventional solvents and rinses can also introduce difficulty through absorption into wafer surface materials. For example, aqueous solutions can be absorbed into a porous matrix, such as that of a porous low-K material. Driving off absorbed aqueous solution from the porous matrix of the low-K material during the drying process can cause physical damage or changes to the low-K material structure or enhance diffusion of contaminants through the low-K material. Physical damage, changes, and contamination of the low-K material can degrade its performance. However, allowing the absorbed aqueous or semi-aqueous solution to remain in the porous matrix of the low-K material can lower the dielectric constant of the low-K material and adversely impact device performance. Due to the difficulty associated with the aqueous or semi-aqueous nature of conventional solvents and rinses, it is desirable to develop an alternative approach for performing the wafer cleaning and rinsing process.
FIG. 2 is an illustration showing a generalized material phase diagram, in accordance with the prior art. The phase of the material is represented as regions of solid, liquid, and gas, wherein the presence of a particular phase is dependent on pressure and temperature. The gas-liquid phase boundary follows an increase in both pressure and temperature up to a point called the critical point. The critical point is delineated by a critical pressure (Pc) and a critical temperature (Tc). At pressures and temperatures beyond Pc and Tc, the material becomes a supercritical fluid.
The supercritical fluid shares the properties of both a gas phase and a liquid phase. The supercritical fluid has near zero surface tension. Therefore, the supercritical fluid can reach into and between small features on the wafer surface without causing the problems associated with the high surface tension of an aqueous or semi-aqueous solution. Also, the supercritical fluid has a diffusivity property similar to a gas. Therefore, the supercritical fluid can get into porous regions of wafer materials, such as low-K dielectric material, without becoming trapped. Additionally, the supercritical fluid has a density similar to a liquid. Therefore, more supercritical fluid can be transported to the wafer in a given amount of time as compared to a gas.
One prior art approach to using supercritical fluid in a wafer cleaning process is to fill a chamber with supercritical fluid and allow a wafer to soak in the supercritical fluid. However, simply filling the chamber with supercritical fluid and allowing the wafer to soak is not sufficient to remove strongly adhering particulate contamination. Furthermore, adding a solvent to the supercritical fluid and allowing the wafer to soak may dissolve some contaminants but is not sufficient to dislodge strongly adhering particulate contamination. Therefore, even with supercritical fluid it is necessary to apply sufficient energy to the particle/wafer interface to dislodge the particulate contamination.
A prior art approach for applying energy in the form of shear force to the particle/wafer interface involves repeatedly filling and flushing the chamber with supercritical fluid. The flow of supercritical fluid over the wafer surface during the flush is intended to impart sufficient shear force to the particle/wafer interface to dislodge the particulate contamination. As particles decrease in size, the linear velocity of the supercritical fluid required to dislodge the particles increases. For example, a supercritical fluid linear velocity of about 100 cm/sec is required to dislodge a particle having a size of about 0.1 micron. Unfortunately, using the fill and flush approach to obtain sufficient supercritical fluid linear velocities at the particle/wafer interface to dislodge smaller particles is difficult, if not impossible. One reason for this is that it is not reasonable to design and operate a chamber which relies on the flushing to impart the necessary linear velocity to the supercritical fluid to cause particulate contamination to be dislodged from the wafer surface. To cause the supercritical fluid to flow at the required velocity during the flush operation, a sufficient pressure drop must exist across the chamber. However, both a pressure greater than Pc and a temperature greater than Tc must be maintained within each region of the chamber during the flush operation to preserve the supercritical phase of the supercritical fluid. Additionally, for the larger particulate contamination which may be removed with the fill and flush approach, multiple fill and flush cycles are required to adequately remove the particulate contamination. Use of multiple fill and flush cycles is time-consuming and not suitable for single-wafer process cycle times.
In view of the foregoing, there is a need for an apparatus and a method for effectively and efficiently using supercritical fluid to remove particulate contamination from a wafer surface. The apparatus and method should avoid the problems associated with relying on high supercritical fluid linear velocities to remove small particulate contamination. The apparatus and method should also avoid the use of extended single-wafer process cycle times.